High-frequency radio observations of the Kühr sample and the epoch-dependent luminosity function of flat-spectrum quasars

¹ Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 2121, Australia e-mail: Roberto.Ricci@csiro.au
² INAF, Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
³ INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40126 Bologna, Italy
⁴ INAF, Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: dezotti@pd.astro.it
⁵ SISSA/ISAS, via Beirut 2–4, 34014 Trieste, Italy

Received: 8 July 2005
Accepted: 6 September 2005

Abstract

We discuss our ATCA 18.5 and 22 GHz flux density measurements of Southern extragalactic sources in the complete 5 GHz sample of Kühr et al. (1981, A&AS, 45, 367). The high frequency (5–18.5 GHz) spectral indices of steep-spectrum sources for which we have 18.5 GHz data (66% of the complete sample) are systematically steeper than the low frequency (2.7–5 GHz) ones, with median , median (), and median steepening , and there is evidence of an anti-correlation of with luminosity. The completeness of 18.5 GHz data is much higher (89%) for flat-spectrum sources (mostly quasars), which also exhibit a spectral steepening: median , median (), and median . Taking advantage of the almost complete redshift information on flat-spectrum quasars, we have estimated their 5 GHz luminosity function in several redshift bins. The results confirm that their radio luminosity density peaks at but do not provide evidence for deviations from pure luminosity evolution as hinted at by other data sets. A comparison of our 22 GHz flux densities with WMAP K-band data for flat-spectrum sources suggests that WMAP flux densities may be low by a median factor of 1.2. The extrapolations of 5 GHz counts and luminosity functions of flat-spectrum radio quasars using the observed distribution of the 5–18.5 GHz spectral indices match those derived directly from WMAP data, indicating that the high frequency WMAP survey does not detect any large population of FSRQs with anomalous spectra.